This invention relates to an adaptive antenna array for mobile communications, and in particular to an adaptive antenna array for use in a mobile receiver in a multipath environment.
Multipath propagation can severely distort the frequency response of a communications channel, especially in a dynamic environment. Multicarrier wireless transmission systems employing orthogonal frequency division multiplexing (OFDM) modulation have become attractive due to their high spectral efficiency and resistance to noise and multipath effects. However, in a moving environment, channel characteristics can change quickly, and additionally, the orthogonality of an OFDM signal can be adversely affected by Doppler spread. Rapidly changing channel characteristics and Doppler spread are prominent problems that limit data transmission speed in a mobile environment.
An adaptive antenna array includes an array of antenna elements connected to a communications receiver and operates by combining the signals received by the antenna elements to adaptively optimize the receive characteristics of the array. By weighting and then summing the multiple antenna signals, the adaptive antenna array can adapt its array pattern to changes in the propagation environment. The adaptive antenna array adjusts the directivity of the antenna adaptively so that the antenna receives the most preferable signal among a plurality of signals that reach the antenna, thereby avoiding the effects of multipath propagation from the same source and noise interference from undesired sources. In a mobile transmission environment, the interference suppression capability of an adaptive antenna array offers the potential to reduce co-channel interference, improve coverage quality, and increase overall system capacity.
Signal processing algorithms for mobile antennas arrays can generally be divided into two major classes, namely time domain algorithms and frequency domain algorithms. An example of a frequency domain algorithm for use in an OFDM system is shown in U.S. Pat. No. 5,973,642 issued Oct. 26, 1999 to Li et al. One drawback with frequency domain algorithms is that they require an FFT engine for each of the separate antennas, which can be cost prohibitive if the adaptive antenna array is to be located in low cost mobile receivers. A time-domain algorithm in an OFDM receiver can be more cost effective as it does not require a full FFT engine for each antenna as the combining is performed prior to transforming the received signals into the frequency domain.
A number of patents and publications have suggested various time domain algorithms for determining the weighting to be applied to signals received by the antenna elements of an adaptive antenna array. However, most of the previously suggested algorithms have been directed towards canceling co-channel interference, rather than eliminating multipath interference and Doppler spread resulting from multipath propagation.
In an article entitled xe2x80x9cCPVL-Algorithm for Phase-only Weighted Adaptive Mobile Radio Antennaxe2x80x9d, Journal of Telecommunications, November/December 1998, the inventor of the present invention proposed an algorithm for a phase only weighted array that efficiently overcomes non-coherent multipath propagation. The CPVL algorithm is also valid for adaptive antenna arrays that use complex weight factors. However, like other time-domain systems, such system fails to perform adequately in coherent multipath conditions, which is disadvantageous given that even in a mobile environment frequently the transmitter and receiver will both be stationary, which may result in a substantially coherent multipath conditions.
In view of the forgoing, there is a need for an efficient, time-domain based adaptive antenna array which can be used with an OFDM receiver for both coherent and non-coherent multipath reception, and which is cost-effective to implement. There is a need for a mobile adaptive antenna array that can efficiently cancel unwanted multipath signals and also suppress interfering signals (such as co-channel interference) at the same time. It is also desirable to have an adaptive antenna array for which explicit knowledge of the array geometry and calibration of the array is not required.
In accordance with the present invention, pseudo random training symbols and/or a constant modulus pilot carrier in OFDM symbols are used to train an adaptive antenna array to cancel unwanted multipath signals and suppress interfering signals. Power variance and power level measurements taken from a plurality of antenna branches during the training symbols and/or constant modulus pilot carrier in OFDM symbols are used to determine appropriate weights to be applied to each antenna branch.
According to one aspect of the invention, there is provided a method for combining a plurality of antenna output signals, each antenna output signal being received from one of a plurality of branches fed from a corresponding plurality of antennas, each radio signal comprising a frame including a pseudo random training symbol and at least one data symbol. The method includes (a) determining a weighting factor for each branch, the weighting factor being determined from power characteristics of the pseudo random training symbol; and (b) combining the antenna output signals from each branch in accordance with the determined weighting factors to form a combined signal. The data symbol may be a multicarrier modulated symbol such as an Orthogonal Frequency Division Modulated (OFDM) symbol. Preferably, the weighting factor for each branch is determined from an average power and power variance determined from the pseudo random training symbol.
According to another aspect of the invention, there is provided a radio frequency receiver including an antenna array device for combining signals received by a plurality of antennas in a communications system where the antennas receive data signals that include a pseudo random training symbol and at least one data symbol. The antenna array device comprises (a) an array controller, arranged to receive the data signals from each of the antennas, for determining a weighting factor for each antenna based on power characteristics of the pseudo random training symbol; (b) a weighting device coupled to each antenna for weighting the data signal received by each antenna according to the weight factor determined for each antenna; and (c) a combiner for receiving and combining the weighted data signals to form a combined data signal. Preferably the array controller determines the weighting factor for each antenna based on an average power and power variance determined from the pseudo random training symbol.
According to a further aspect of the invention, there is provided a method for steering an adaptive antenna array that includes a plurality of antennas in a communications system where the antennas receive and output data signals that include a training symbol and at least one data symbol, the training symbol including a first pseudo random subsymbol and a second subsymbol that is a copy of the first subsymbol. The method includes (a) separating training symbol information from interfering noise signal information based on autocorrelation properties of the training symbol; (b) determining a weighting factor for the signals received by each antenna, the weighting factor being determined from power characteristics of the separated training signal information; and (c) combining the signals from each antenna in accordance with the determined weighting factors to form a combined signal. Preferably, the output data signals include a null symbol, the method including a step of measuring interfering signal characteristics during the null symbol and, based on the interfering signal characteristics, selecting which of a plurality of predetermined algorithms to use to determine the weighting factor for signals received by each antenna.
According to still a further aspect of the invention, there is provided a method for combining a plurality of antenna output signals, each antenna output signal being received from one of a plurality of branches fed from a corresponding plurality of antennas, each radio signal comprising a plurality of OFDM symbols including a constant modulus pilot carrier. The method includes (a) determining a weighting factor for each branch, the weighting factor being determined from power characteristics of the constant modulus pilot carrier; and (b) combining the antenna output signals from each branch in accordance with the determined weighting factors to form a combined signal.